Twin Pregnancy Calculator — Week 20: Di/Di, Mo/Di & Mo/Mo
You are 20 weeks pregnant with twins. Twin gestation deviates meaningfully from singleton pregnancy from week 20 onwards — chorionicity, growth discordance and delivery timing all change. Here is the exact week-20 twin picture.
Twin Pregnancy Calculator
Enter the first day of your last menstrual period (LMP). EDD = LMP + 280 days.
Baby at week 20
Per twin, week 20 development: Halfway point. Fetus swallows amniotic fluid — practices digestion. Sex confirmed on anatomy scan.
Mother's body — what changes
Twin pregnancy at week 20 differs — expect: Fundus reaches the navel. Weight gain 4–6 kg by now is expected in singleton pregnancy.
Nutrition & supplements
Add ~340 kcal/day above pre-pregnancy baseline. Prioritise iron (27 mg/day), calcium (1000 mg), omega-3 DHA (200–300 mg) and 71 g protein. Continue prenatal vitamin. For twin gestation add an extra 300 kcal on top (total ~600 second trimester, ~750 third), 50% more iron, and split hydration to 3–3.5 L/day.
Safe exercise
Continue 150 min/week. Avoid supine (flat-on-back) exercises after week 20 — they compress the vena cava. Swimming and prenatal Pilates are ideal now. For twins reduce intensity 20% earlier — bed rest is not routinely indicated but modified activity from week 24 is prudent.
Doctor visit checklist — week 20
- ✓Anatomy scan completed — save images and report
- ✓Chorionicity confirmed at 10–14 weeks (Di/Di, Mo/Di, or Mo/Mo)
- ✓Growth scans every 4 weeks (Di/Di) or every 2 weeks (Mo/Di, TTTS surveillance)
- ✓Maternal-fetal medicine (MFM) referral by week 20
Warning signs — call your provider immediately
- ⚠Heavy bleeding (soaking a pad in an hour)
- ⚠Severe abdominal pain that does not ease
- ⚠Fever above 38°C / 100.4°F
- ⚠Sudden gush of fluid (early rupture of membranes)
- ⚠Absent fetal movement for 12+ hours after 24 weeks
- ⚠Severe headache + visual disturbance (preeclampsia)
Week 20 — Frequently asked questions
How accurate is the Twin Pregnancy Calculator at week 20?
The calculator uses standard gestational-age arithmetic (LMP + 280 days for standard; egg retrieval + 266 days for IVF day-5). Ultrasound crown-rump length between weeks 8–13 is more accurate (±5 days) than LMP — adjust EDD accordingly if the scan differs by more than 7 days.
What size is my baby at week 20?
Approximately the size of a banana — length 25.6 cm and weight 300 g per twin (combined weight roughly 1.8× singleton for well-nourished mothers). These are median values from SMFM, ACOG twin guidelines and NICE multiple pregnancy references.
Which symptoms are normal in week 20?
Fundus reaches the navel. Weight gain 4–6 kg by now is expected in singleton pregnancy. Every pregnancy is different — call your provider if anything on the warning-signs list appears.
Should I have an ultrasound at week 20?
Anatomy scan completed — save images and report. Routine scan windows are: dating (10–13w), anatomy (18–22w), growth (28–32w).
How much weight should I gain by week 20?
Approximately 5.1 kg cumulative if you started at a normal BMI (IOM guideline). Underweight mothers gain more; overweight mothers less. Twin gestation targets 16–24 kg total (higher than singleton 11–16 kg).
Is my twin pregnancy Di/Di, Mo/Di or Mo/Mo?
Chorionicity is confirmed on the 10–14 week ultrasound by looking at the membrane between twins — 'lambda sign' = Di/Di (safest), 'T sign' = Mo/Di (requires TTTS monitoring every 2 weeks), no membrane = Mo/Mo (highest risk, delivery by 34 weeks).
When are twins usually delivered?
Di/Di twins: 38 weeks. Mo/Di twins: 36–37 weeks. Mo/Mo twins: 32–34 weeks by planned C-section. About 60% of twins deliver preterm (<37 weeks) spontaneously.
Content aligned with SMFM, ACOG twin guidelines and NICE multiple pregnancy. Reviewed by Dr. Maria Rehman, MD (OB/GYN). This page is educational and does not replace personal medical advice — call your obstetrician for any concern.